1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on an image forming medium, particularly to an image forming range varying system and a method of varying an image forming range in which an image is formed in accordance with a shape of the conveyed image forming medium.
2. Description of the Related Art
In general, in an image forming apparatus represented by a printer or a copying machine, an image forming medium (hereinafter referred to as the medium) is conveyed, and an image is formed during the conveyance. For example, in the image forming apparatus of an ink jet system, ink is jetted to a medium (e.g., printer sheet) conveyed to a conveying unit such as a platen from a head nozzle of an ink head, and the image is formed. At this time, when a shape of the medium (forming range) does not agree with that of the formed image (forming range), the ink is jetted to the conveying system to convey the medium in a portion in which the shapes do not agree with each other, and this is a cause for dirt.
Therefore, the medium passes a predetermined position in a medium supply system and a medium conveying system at a predetermined timing, and the medium is obliquely inclined with respect to a conveying direction (X-axis direction or sub-scanning direction), and hereof, have to be prevent being so obliquely inclined of the medium. For example, in Jpn. Pat. Appln. KOKAI Publication No. 2001-96874, a technique has been described in which a detection unit to detect an end portion in image receiving paper P is disposed, a recording head to form the image is controlled in response to a detection signal (presence region information) of the detection unit, and the jetting of the ink is stopped in a portion where the image receiving paper P does not exist to prevent dirt on the periphery.
As described above, the protruding image ink sticks to the conveying unit including the platen, and the back surface of the medium to be treated next is made dirty. Then, according to a configuration shown in the Jpn. Pat. Appln. KOKAI Publication No. 2001-96874, an image recording apparatus is described having an ink head in which a light source for detection, formed in a range longer than a transverse width of image receiving paper P, a plurality of light receiving elements to receive reflected light from the image receiving paper P by the light source for the detection, and a plurality of ink nozzles are integrally formed in order from an upstream side of the conveying direction in the conveying direction (sub-scanning direction) of the image receiving paper P (medium).
The image recording apparatus has such a constitution that the ink is not jetted from the ink nozzle which has not detected the image receiving paper P by detection signals (presence region information in the end portion of the image receiving paper P) of the plurality of light receiving elements. Prior to the ink-jet timing, a delay time t is determined from the speed at which the image receiving paper P is conveyed and the distance between the ink nozzle and the point on the paper P, at which the light beam emitted from the detecting light source is applied to the paper P. The application of ink from the nozzle is delayed by the time t thus determined. Note that the distance is one measured in the sub-scanning direction.